As used herein, the term EMI should be considered to refer generally to both EMI and radio frequency interference (“RFI”) emissions, and the term electromagnetic should be considered to refer generally to electromagnetic and radio frequency.
During normal operation, electronic equipment generates undesirable electromagnetic energy that can interfere with the operation of proximately located electronic equipment due to EMI transmission by radiation and conduction. The electromagnetic energy can be of a wide range of wavelengths and frequencies. To minimize the problems associated with EMI, sources of undesirable electromagnetic energy may be shielded and electrically grounded. Shielding is designed to prevent both ingress and egress of electromagnetic energy relative to a housing or other enclosure in which the electronic equipment is disposed. Since such enclosures often include vent openings and gaps or seams between adjacent access panels and around doors, effective shielding is difficult to attain because the gaps in the enclosure permit transference of EMI therethrough. Further, in the case of electrically conductive metal enclosures, these gaps can inhibit the beneficial Faraday Case Effect by forming discontinuities in the conductivity of the enclosure which compromise the efficiency of the ground conduction path through the enclosure. Moreover, by presenting an electrical conductivity level at the gaps that is significantly different from that of the enclosure generally, the gaps can act as slot antennae, resulting in the enclosure itself becoming a secondary source of EMI.
Specialized EMI gaskets have been developed for use in shielding small gaps in electronic enclosures. These include, but are not limited to, metal spring fingers, wire mesh, fabric-over-foam, and conductive elastomers. To shield EMI effectively, the gasket should be capable of absorbing or reflecting EMI as well as establishing a continuous electrically conductive path across the gap in which the gasket is disposed.
One particularly challenging shielding issue on electronic enclosures is the ventilation opening. In many enclosures, openings that are much larger than gaps along seams and I/O ports are intentionally placed in the enclosures to facilitate the removal of heat. Without EMI shielding, the openings represent huge EMI leakage points. One common approach to shielding these areas is to use ventilation panels, also known as vent panels. Traditional vent panels consist of a metallic honeycomb material mechanically assembled into a stiff metallic frame. This assembly is then fastened to the enclosure with some type of EMI gasketing installed along the enclosure/vent panel interface. The vent panels can be used in the as-manufactured state or they can be plated. Lower cost vent panels, which are usually made of aluminum honeycomb, provide lower levels of shielding effectiveness and are not structurally robust. In applications that require a very robust vent panel, which also provides very high level of shielding effectiveness, steel or brass honeycomb is often used. These products, however, are much more expensive.
A key attribute of any vent panel is the ease of airflow through the honeycomb, because cooling capability is directly related to volume of airflow per unit of time. Also, in traditional vent panels, electrical contact is made by mechanically crimping the metal frame against the honeycomb material, such that the metal frame causes an indentation of the honeycomb material along the edge of the frame. This insures good electrical contact as long as the frame is not subjected to severe bending or torque.
Enclosures for electronic equipment use airflow to remove heat from the enclosures. Honeycomb filters can be installed in an opening on the enclosure to serve as ventilation panels. In addition, honeycomb filters also provide EMI shielding. Examples of commercially available honeycomb filters are designated “Commercial Honeycomb Ventilation Panels” and “BE 11 ALU-HONEYCOMB FILTERS” air ventilation panels manufactured by Laird Technologies, Inc. (f/k/a Instrument Specialties Co. and Advanced Performance Materials). Another example of commercially available honeycomb filters are designated RF CORE honeycomb cores manufactured by R & F Products, located in San Marcos, Calif. Other similar commercially available ventilation panels are manufactured by Tecknit located in Cranford, N.J., and Chomerics located in Woburn, Mass.
As shown in FIG. 1, commercially available vent panels 10 typically include a honeycomb substrate 12 and a frame 14. The honeycomb substrate 12 is typically made out of very thin strips of corrugated aluminum. In most cases glue, spot welds, or other attachment methods are used to hold the honeycomb substrate 12 together. Piercings are often made between the aluminum layers to improve electrical conductivity. The electrically conductive aluminum honeycomb substrate 12 may optionally be covered with a conductive layer to enhance electrical conductivity across the honeycomb substrate 12. Some examples of conductive layers are an aluminum chromated layer or a tin plated layer. These coatings may also be added to enhance corrosion resistance.
As shown in partial cross-section in FIG. 2, the frame 14 is crimped onto the honeycomb substrate 12. The frame 14 includes solid pinch fingers 16 to grip the honeycomb substrate 12. The frame 14 and honeycomb substrate 12 are in electrical communication with each other so EMI emissions captured by the honeycomb substrate 12 can be transferred from the honeycomb substrate 12 to the frame 14 and ultimately to the electronic enclosure. The design of these pinch fingers 16 results in a line contact between the frame 14 and the honeycomb substrate 12. This feature can make the vent panel 10 susceptible to localized EMI leakage if twisting and jarring of the vent panel 10 degrades that contact area. In addition, the need for the pincher fingers 16 in the metal extrusion limits how narrow the frame 14 can be manufactured, typically not less than 0.25 inch wide.
As shown in FIG. 3, the vent panel 10 is installed in an opening 18 formed in an enclosure 20 for electronic equipment. An EMI gasket 22 is attached to the vent panel 10 about a perimeter thereof to seal EMI leakage paths between the enclosure 20 and the vent panel 10.
The vent panel 10 allows air to flow through the honeycomb substrate 12 to ventilate and cool the electronic equipment inside the enclosure 20. As electronic applications achieve higher clock speeds, and as electronic components are more compactly packed in the enclosure 20, the heat generated within the enclosure 20 increases, necessitating higher airflow. However, airflow through the vent panel 10 is limited by the pressure of the frame 14. Depending on the design of the vent panel 10, the presence of the frame 14 can reduce airflow through the opening 18 by about 5% to 15% or more. Traditional frames, with the pincher finger feature, greatly limit the ability to increase vent panel airflow due to the minimum width requirements of the frame material.
Another problem with commercially available vent panels 10 is that they are typically made of aluminum, which is not very resilient and therefore subject to damage. The lack of resiliency results in plastic deformation of the honeycomb filter due to impacts that can be encountered during assembly and field use. To ensure proper airflow after damage, cells of the honeycomb have to be reworked. The rework process is time consuming, requiring the deformed aluminum strips to be bent to open the cells. Even with rework, there is typically degradation of flow through the vent panel 10. In addition, the rework often results in an aesthetically undesirable appearance. There is a need for a honeycomb filter with improved airflow capability and improved durability.